Correction device using magnetic elements for correcting unhomogeneities of the magnetic field in a magnet

ABSTRACT

Corrector device for correcting unhomogeneities of the magnetic field produced by a magnet, said device applying a principle of correction of the field by means of one of a plurality of magnetic elements in a manner which is much simpler and quicker than in the prior art. To this effect, the corrector device of the invention comprises premachined openings which are intended to house a magnetic element and comprises means to maintain confound a longitudinal axis of the magnetic element and a longitudinal axis of the opening, independently of the value of the cross-section of the magnetic element.

The present invention is due to the collaboration of the ServiceNational des Champs Intenses (Director, Mr. Guy Aubert), and its objectis a correction device using magnetic elements to correctunhomogeneities of the magnetic field produced by a magnet. Itparticularly concerns means for the assembly and positioning of one ormore of these magnetic elements. The invention can be applied especiallyin the medical field where magnets are used in methods of nuclearmagnetic resonance imaging. However, it can find application in otherfields, notably in that of scientific research, where intense fieldsproduced by magnets are used.

Magnetic resonance is a phenomenon of oscillation of the magnetic momentof the nuclei of the atoms or molecules of a body, at a frequency thatdepends on the intensity of a magnetic field in which this body isbathed. It follows that if the intensity of the magnetic field varies,the frequency of the phenomenon of resonance varies too. Hence, fortechnological and technical reasons, it is of the utmost importance thatthe field produced by the magnet should be very homogeneous in its zoneof interest. The requisite homogeneity is usually of the order of someparts per million in the medical field, or even some parts per billion(1000,000,000) in the scientific field. To achieve this end, it issought to construct magnets with a field that is as perfectlyhomogeneous as possible.

Unfortunately, irrespectively of the care that might be taken inconstructing the magnets, they are never as perfect as the theory thathas led to their design. Besides, even if this fault can be eliminated,the magnet to be used should be placed physically in a given location.Now, none of the regions of space, in an industrial or urbanenvironment, is totally free of magnetically disturbing elements. Theresult thereof is that once the magnet is installed on the site, thefield that it produces in a zone of interest has non-homogeneities whichmust be corrected.

The principle of the correction of non-homogeneities of the fields isthat of superimposition: coils, magnetic pieces, or any other meansenabling a correction of the imperfections of the main field and theobtaining of a homogeneous total field in the zone of interest are addedon. A known method to correct the non-homogeneities of the magneticfield produced by a magnet consists in the use of magnetic elements suchas magnetizable bars, for example made of soft iron, which are placed inthe environment of the magnet and exert their influence in a zone ofinterest of the magnet so as to correct the non-homogeneities of thefield in this zone.

A method of this type is commented on in an article by D. I. Hoult andD. Lee, "Shimming a Superconducting Nuclear Magnetic Resonance ImagingMagnet With Steel" in Revue Sci. Instrum., January 1985, pp. 131 to 135.This article concerns, particularly, a nuclear magnetic resonanceimaging apparatus. The magnet has the shape of a circular cylinder,within which the zone of interest is located. The center of this zone ofinterest is placed on the axis of the magnet. The magnetic fieldproduced by the magnet is within this magnet, substantially parallel tothe axis of the magnet. This document gives a detailed description of amethod of computation which can be used to determine the dimensions andposition of one or more magnetic bars, made of soft steel, around theaxis of the magnet, as a function of a point of the volume of interestwhere a non-homogeneity of the field has to be corrected. The length ofthese correction bars is parallel to the axis of the magnet.

A method such as this, despite its relative complexity, enables theefficient correction of the possible non-homogeneities of the magneticfield in the volume of interest, by the placing of one or more magneticbars, for which the section, length and position with respect to thevolume of interest are determined on the basis of computations that areknown and are, for example, of the type indicated notably in theabove-mentioned article. But, as known, this method has the drawback ofbeing hard to apply in the context of the industrial scale fabricationof magnets. In effect, especially in view of, firstly, manufacturingtolerances which may introduce differences, between one magnet andanother, in the magnetic fields produced by these magnets and, secondly,in view of elements which, on the site of their installation, coulddisturb the magnetic field, it is necessary to redefine, at the sametime, the length, cross section and position of the magnetic correctionbars when the magnet is already installed on the site, i.e. the magneticbars should be cut and positioned to order for each application. Thedocument EP-A-0 222 281 describes a simplification of this problem inthat it provides for openings designed to receive longitudinal, magneticcorrection elements. However, the positioning of these elements has notbeen thoroughly considered, so that the correction provided is not as asefficient as desired.

The present invention concerns a device for the correction ofnon-homogeneities of the magnetic field produced by a magnet, enablingthe application of the principle explained above for the correction ofthe field by one or more magnetic bars and, in particular, it has theobject of facilitating the implementation of this principle on the siteof installation of the magnet, notably in order to make it applicable inthe context of industrial-scale production. This is obtained with thecorrection device according to the invention by a novel arrangementwhich is achieved during a stage for the construction or assembly of themagnet and which, firstly, provides for the possible positions of one ormore magnetic correction elements or magnetic bars and, secondly, can beeasily used on the installation site to adjust the degree of influenceof the magnetic bar or bars.

According to the invention, a device for the correction ofnon-homogeneities of a magnetic field in a magnet, comprising at leastone elongated magnetic element, the length and cross section of whichare defined according to the correction to be made, a support bearingthe magnetic element in an opening in the vicinity of the field to becorrected, is characterized in that the opening is formed by a locationreserved during the building of the support, the opening having a crosssection and length equal to or greater than, respectively, a maximumlength and a maximum cross section of the magnetic element, and in thatit further comprises means to keep the longitudinal axis of the magneticelement substantially identical with the longitudinal axis of theopening.

The invention will be better understood from the following description,given as a non-restrictive example, and the two appended figures, ofwhich:

FIG. 1 represents, schematically, through a view in perspective, anuclear magnetic resonance imaging apparatus incorporating anon-homogeneity correction device according to the invention;

FIG. 2 is a longitudinal sectional view of the MRI apparatus and enablesthe depiction of the characteristic details of the correction deviceaccording to the invention.

FIG. 1 shows a non-restrictive example of an application of a correctiondevice 10, according to the invention, to the correction of fieldnon-homogeneities of a nuclear magnetic resonance imaging or MRIapparatus 1, the depiction of the MRI apparatus I being restricted tothe elements needed to understand the invention. The MRI apparatus 1 hasthe general shape of a circular cylinder and comprises a magnet 2, alsoof a cylindrical shape, the axis Z of which is also the longitudinalaxis of the MRI apparatus 1, the magnet 2 being formed, for example, byan arrangement of electrical coils (not shown) of a known type. Themagnet 2 produces a magnetic field B_(O) which is oriented, inside themagnet 2, along the axis Z of the latter. Along the axis Z of themagnet, an inner free space 4 forms a tunnel designed to receive apatient (not shown) to be examined. According to a standard arrangement,the MRI apparatus 1 has different elements around the tunnel 4. Theseelements appear, in FIG. 1, at one end 11 of the MRI apparatus 1 inseveral coaxial layers 5, 6, 7, 2: The first layer 5, which is closestto the axis Z of the magnet, represents the place occupied by standardgradient coils or radiofrequency antennas (not shown). The second layer6, which surrounds the first layer 5, symbolizes the place occupied bystandard gradient coils (not shown). In the non-restrictive exampledescribed, a third peripheral layer 7 represents the place occupied byfield correction coils of a known type (not shown) and the last layer 2represents the magnet. The correction coils 7 enable corrections to bemade in a standard way in the non-homogeneities of the field B_(O) inthe magnet 2, in a zone of interest (not shown), the center 0 of whichis on the axis Z of the magnet, in adjusting the intensity of thecurrent that is applied to them. These coils 7 can be used to make veryfine corrections but, in certain cases, their efficiency is reduced bythe fact that their position is frozen.

The MRI apparatus 1 further includes the correcting device 10, enablingthe correction of the non-homogeneities of the field B_(O) by means ofat least one magnetic element A1 which may equally well be of themagnetizable type (temporary magnetization), i.e. made of soft magneticmaterial such as, for example, of soft ferrite or soft iron, or of themagnetized type (permanent magnet). In the non-restrictive exampledescribed, a single magnetic element A1, having the shape of a bar, isshown in FIG. 1. It is designed to be housed in an opening C1 in the MRIapparatus 1 in order to act on the field B_(O), particularly in the zoneof interest which is located in the internal space 4 and which has itscenter 0 on the axis Z of the magnet. As explained in the rest of thedescription, the correction device 10 may nonetheless include severalopenings and several magnetic elements or magnetic bars, especially inthe case of an application to an MRI apparatus.

The correction device 10 further includes means 20, D1, D2, enablingprecise positioning of the magnetic bar A1, with respect, for example,to the center of interest 0. In the non-restrictive example described,the opening C1 has a longitudinal axis 30, parallel to the axis Z of themagnet, and it is located inside the magnet 2, i.e. between the knowntype of correction coils represented by the third layer 7 and the secondlayer 6 which represents the above-mentioned gradient coils, the bar ormagnetic element A1 being, in the non-restrictive example described, ofthe magnetizable type. The MRI apparatus 1 thus forms the support of thebar or magnetic element A1, but the opening C1 could equally well bemade at another position, for example externally with respect to themagnet 2, and can be made in a different support (not shown).

According to a characteristic of the invention, the opening C1 is madeby construction, i.e. it is formed at a pre-determined position Pl by alocation reserved for this purpose during the building of the MRIapparatus 1.

The length L1 and/or the section S1 of the magnetic bar A1 are liable tovary from one magnet to another, depending on the non-homogeneity of thefield B_(O) to be corrected. To take this possibility into account, theopening C1 is given, firstly, a second cross section S2, equal to orgreater than the maximum cross section that the magnetic bar A1 may haveand, secondly, a second length L2 equal to or greater than the maximumlength that a magnetic bar or magnetic element A1 can have.

According to another characteristic of the invention, the positioningmeans have at least one radial shim 20 in which the magnetic bar A1 isengaged. The radial shim or shims 20 form means to keep the magnetic barA1 in the opening C1 so that the longitudinal axis 30 of this opening isidentical with a longitudinal axis 8 of the magnetic bar A1.

The cross sections S1, S2 of the magnetic bar A1 and the opening C1 mayhave any shape, for example square or rectangular or circular, as in theexample shown in FIG. 1, where the circular section S1 of the magneticbar A1 has a first diameter d1 smaller than the the second diameter d2of the circular section S2 of the opening C1. The maximum possiblesection of a magnetic element or bar A1 corresponds to the diameter d2of the opening C1, and the second length L2 of this opening representsthe maximum possible length of a magnetic bar A1. In the non-restrictiveexample described, the second length L2 of the opening C1 corresponds tothe length of the MRI apparatus 1, so that the two ends 9, 12 of theopening C1 are accessible.

The means 20, to keep the longitudinal axes 8, 30 of the magnetic bar A1and the opening C1 aligned, are made of a material that has no effect ona magnetic field, such as epoxy resins or fiber glass for example. Thesemeans can be formed in different ways, the essential point being thatthe magnetic bar A1 should be kept in the opening C1 so that theirrespective longitudinal axes 8, 30 are substantially identical,irrespectively of the shapes of the cross sections S1, S2 and,especially, when the section S1 of the magnetic bar A1 is smaller thanthat of the opening C1. In the non-restrictive example described, thesemeans are formed by radial shims 20, each formed by a circular disc withan axial opening 19 having the same diameter d1 as the magnetic bar usedand having an external shape 18, represented by the external diameter,that corresponds to the second diameter d2 of the opening C1.

The positioning means further have longitudinal shims D1, D2, designedto be mounted in the opening C1, on either side of the magnetic bar A1,in order to block this bar in the position that it should occupy alongthe axis Z of the magnet. Thus, for example, if the length L1 that thebar A1 must have is 0.31 m, and if this length L1 has to be centeredwith respect to the center of interest 0 which is, itself, in the centerof the MRI apparatus 1, the second length L2 of the latter being, forexample, two meters: the first and second longitudinal shims D1, D2 haveequal lengths L3, L4, of 0.845 meters each, i.e. the addition of thelengths L1, L3, L4 corresponds to the second length L2 of the openingC1; if the position of the magnetic bar A1 has to be off-centered withrespect to the center of interest 0, closer to the first end 9 of theopening C1, for example, the length L3 of the first longitudinal shim D1is diminished and the length L4 of the second longitudinal shim D2 isincreased.

According to another characteristic of the invention, the correctiondevice 10 has a plurality of openings C1, C2, . . . , Cn, which aresimilar to the first opening C1 and are made by construction like thislatter opening during the assembling of the MRI apparatus 1. It shouldbe noted that it is relatively easy to set aside locations designed forthe openings C1, . . . , Cn in placing, at these locations, sheaths ortubes made of non-magnetic material, for example fiber glass. Theopenings C1, . . . Cn are arranged symmetrically in one and the samecircle with a radius r, around the axis Z of the magnet, and havelongitudinal axes 30 parallel to the latter. Each of these openings C1to Cn may or may not have a magnetic bar A1, under the same conditionsas those described with respect to the first opening C1 i.e. for eachmagnetic bar placed in one of these openings C1 to Cn, a set oflongitudinal shims is used, similar to the longitudinal shims D1, D2,and it is possible to use radial shims 20 adapted to the diameter d1 ofthe magnetic bar when the latter has a first diameter d1 smaller thanthe second diameter d2 of the opening C1, . . . , Cn used.

In the non-restrictive example of the description, the openings C to Cnare placed at pre-determined angular positions P1, . . . , Pndistributed around the axis Z of the magnet at angles α of 30° , i.e. 12openings have thus been made. However, in the spirit of the invention,the number of these openings may be different, as also their angularposition P1 to Pn and their distance from the axis Z of the magnetrepresented by the radius r.

The magnetic field B_(O) having been measured in a manner known per se,the non-homogeneities of the field B_(O) may be corrected by one or moremagnetic bars A1, . . . , An the number N of which is limited solely bythat of the openings C1 to Cn. After having determined, either by testsor by means of a software program applying standard computations, foreach magnetic bar, its angular position P1 to Pn corresponding to one ofthe openings C1 to Cn, its length L1, its cross section S1 and itslongitudinal position along the axis Z of the magnet, each magnetic baris positioned in the opening C1 assigned to it, and the number ofmagnetic bars could be smaller than that of the openings.

This arrangement of the correction device 10 according to the inventionenables application of the principle of correction of thenon-homogeneities of the field B_(O) by one or more magnetic bars ormagnetic elements A1, AN far more simply and quickly than in the priorart, notably through the existence of the pre-machined openings C1 toCn, and through the use of the longitudinal shims D1, D2 and the radialshims 20 which facilitate the precise positioning of the magnetic barsA1, . . . . , AN. It must be noted that the setting and the computationsare considerably simplified because the longitudinal axis 8 of amagnetic bar A1, AN always keeps one and the same known positionirrespectively of the dimension of the cross section S1 of the magneticbar, this known position being that of the longitudinal axis 30 of theopening C1 to Cn in which the magnetic bar is engaged.

According to another characteristic of the invention, the length L1 ofeach magnetic bar A1, AN can be formed by two or more magnetic sections,placed end to end, as shown in FIG. 2, in order to facilitate, inpractice, the adjustments made on the installation site.

FIG. 2 shows the MRI apparatus in a longitudinal sectional view alongthe axis Z of the magnet, and shows in a sectional view, as an example,the second opening C2 and the eigth opening C8 which are shown in FIG. 1and are opposite with respect to the axis Z of the magnet.

The second and eighth openings C2, C8 are formed by the interior of atube 50, and respectively contain a second magnetic bar and a thirdmagnetic bar A2, A3, each placed between a first longitudinal shim and asecond longitudinal shim D1, D2. The length L1 required for the secondbar A2 may be obtained by a magnetic bar cut as a single piece or, as inthe non-restrictive example shown in FIG. 2, by several magneticsections a1, a2, a3, placed end to end so as to form a single magneticbar A2 having the desired length L1. The first, second and thirdsections a1, a2, a3 may have one and the same length or, as in theexample of FIG. 2, they may have different elementary lengths 11, 12,13, for example 6, 10 and 15 cm. It is thus possible to make magneticbars of different lengths in combining sections a1, a2, a3 . . . , anhaving pre-determined elementary lengths l1, l2, . . . , ln. This mayexist for each section S1 value that may be had by a magnetic bar A1,A2, A3, . . . , AN, so that an operator responsible for the setting caneasily have the necessary parts available. Furthermore, in the mostcommon example, where the position and dimension of the magnetic bars A1to AN are determined by means of a computation program, which takes intoaccount the pre-determined angular position P1 to Pn of the existingopenings C1 to Cn, this program can also take into account the value ofthe different elementary lengths l1, l2, l3, . . . , ln, as well as thedifferent values of the section S1 or first diameter d1 in which thesesections al to an are available. It must be noted that this firstdiameter d1 may vary in steps of 1 mm until the maximum diameter whichcorresponds to the second diameter d2 of an opening C1 to Cn and mayreach, for example, 15 mm.

According to another characteristic of the invention, a magnetic bar maybe formed by one or more separate magnetic sections, as shown in FIG. 2by the third magnetic bar A3 contained in the eighth opening C8. In thenon-restrictive example described, the third magnetic bar A3 is formedby a fourth magnetic section and a fifth magnetic section, a4, a5,separated by a separating shim 40, with which they form a set, the totallength Lt of which is centered with respect to the center of interest 0.However, to obtain the desired correction, the third magnetic bar A3 mayalso be formed by a greater number of magnetic sections a1, a2, a3, a4,a5., . . . , an separated by one or more separating shims 40, andfurther forming a set, the length Lt of which may be off-centered withrespect to the center of interest 0. The separating shims 40, as well asthe longitudinal shims D1, D2, are made of fiber glass or of any othermaterials that exert no influence on a magnetic field.

The correction device 10 according to the invention thus enables the useof any number N of magnetic bars A, A2, . . . , AN of variedcomposition, this number being limited by the number of openings C1 toCn.

Although it has been described with reference to the magnet 2 of anuclear magnetic resonance imaging apparatus, of the type used inmedicine, and in which several correction magnetic bars are generallynecessary, the correction device 10 according to the invention can alsobe applied to the case of magnets of a different type. In particular, itis possible to use the arrangement of the correction device of theinvention by placing one or more magnetic bars outside a magnet, inwhich case the magnetic bars are formed not by magnetizable parts (madeof soft iron) but by parts that have been previously and permanentlymagnetized, and are made of a material for permanent magnets, namely ahard magnetic material (an alloy of rare earths and cobalt for example).

I claim:
 1. A device for the correction of non-homogeneities of amagnetic field (B_(O)) in a magnet, comprising at least one magneticelement, the length and cross section of which are defined according tothe correction to be made, said magnetic element being contained in afirst opening formed in a support located in the vicinity of the fieldto be corrected, characterized in that the opening is formed in thesupport by a location reserved during the construction of the support,the opening having a length and a cross section greater than,respectively, the maximum length and the maximum cross section of themagnetic element, and in that it further comprises means disposed insaid opening to keep the longitudinal axis of the magnetic elementsubstantially identical with the longitudinal axis of the opening. 2.Correction device according to claim 1, characterized in that itcomprises a plurality of openings similar to the first opening andpre-machined.
 3. Device according to claim 2, characterized in that thenumber of magnetic elements is equal to or smaller than the total numberof openings.
 4. Correction device according to claim 1, characterized inthat the means to keep the two longitudinal axes substantially identicalinclude at least one radial shim having an axial opening in which thereis engaged a magnetic element and which has a cross section that issubstantially the same as that of the cross section of the magneticelement.
 5. Correction device according to claim 4, characterized inthat it further comprises at least first and second longitudinal shimsbetween which said magnetic element is contained in said first opening,the addition of the lengths of said longitudinal shims with the lengthof said magnetic element corresponding to the length of said firstopening.
 6. Correction device according to claim 5, characterized inthat the first and second longitudinal shims have equal or differentlengths depending on the position of a magnetic element along thelongitudinal axis of an opening.
 7. Correction device according to claim1, characterized in that said magnetic element is formed by at least twomagnetic sections joined end-to-end.
 8. Correction device according toclaim 7, characterized in that the length of a magnetic element isobtained by the addition of magnetic sections having differentelementary lengths.
 9. Correction device according to claim 7,characterized in that a magnetic element is formed by at least twomagnetic sections separated by a separating shim made of magneticmaterial.
 10. Correction device according to claim 1, characterized inthat the magnet has the shape of a circular cylinder, and in that thelongitudinal axis of the at least one opening is parallel to an axis ofthe magnet.
 11. Correction device according to claim 10, characterizedin that the openings and the magnetic elements are arranged inside themagnet.
 12. Correction device according to claim 10, characterized inthat the openings are arranged around the axis of the magnet inpre-determined angular positions.
 13. Correction device according to oneof the claims 1 to 4, characterized in that the at least one magneticelement is made of soft magnetic material.
 14. Correction deviceaccording to claim 13, characterized in that the magnetic elements aremade of soft iron.
 15. Correction device according to claim 13,characterized in that the magnetic elements are made of soft ferrite.16. Correction device according to one of claims 1 to 4, characterizedin that the at least one magnetic element is made of hard magneticmaterial, previously and permanently magnetized.
 17. Correction deviceaccording to claim 1, wherein the magnet constitutes the magnet of anuclear magnetic resonance imaging apparatus.